1. Field of the Invention
The invention relates to a tuner for tuning an input signal, having an input for receiving the input signal, a UHF section coupled to the input for handling UHF signals, a VHF section coupled to the input for handling VHF signals, the UHF section and the VHF section being coupled, with outputs, to a mixer oscillator stage which comprises UHF oscillation means with a UHF tuning circuit, and VHF oscillation means with a VHF tuning circuit, said mixer oscillator stage being coupled to an output of the tuner. The invention further relates to a mixer oscillator stage.
The invention further relates to a receiver, more particularly, but not exclusively to a television receiver.
2. Description of the Related Art
Such a tuner is used in, for example television receivers. For such a tuner, different solutions are known. As is known, television signals comprise signals in the so-called UHF and VHF band, which are tuned with a UHF section and a VHF section. Tuning can, for example be accomplished by the change in capacitance with an applied dc voltage to varicap diodes. One diode is used in each tuned circuit. Tuning the signals of the UHF band can be covered by a single varicap diode, whereas the tuning of the signals of the VHF band has to be split into at least two ranges and at least two varicap diodes are necessary. The most straight forward solution is to use a so-called 3-band concept (UHF, VHFL and VHFH) having, for each band, its own path: a tuned input circuit, a so-called RF amplifier and a bandpass filter. The UHF, VHFL and VHFH sections are then followed by a mixer oscillator (and PLL) stage.
As the above solution is quite complex, nowadays, to reduce the costs of three separate bands, a switching between the low and high VHF signals (channels) is used. A solution to perform this is to simply short-circuit, with a switching diode, a part of the tuning coil in the relevant resonant circuit to change the tuning frequency range. To obtain the off state of the switching diode, it is well known in the art to supply a negative voltage to the anode of the switching diode. Further, it is known that when the negative voltage is not connected to the switching diode, the switching diode in the oscillator circuit operates as a detector and provides a negative voltage for the other tuned circuits.
See for example "Television and Audio Handbook, page 9.22, Figure. 9.15".
One of the disadvantages of the use of a diode detector is that a good switching diode is a poor detector. Sometimes the switching diode for the oscillator circuit is selected using a so-called detector test.